The invention relates to continuous production of granulates from a solid in a fluidized bed of material where the solid is sprayed into or onto the fluidized bed of pregranulated solid.
A known method has the solid to be granulated, in the form of a solution or suspension, sprayed into or onto the bed of material consisting of pregranulated solid. The bed is fluidized by an injected drying gas of suitable temperature and a corresponding quantity of finished granulates is discharged from the bed of material. The drying gas is drawn off separately from the finished granulates but together with the resultant water vapor. Various forms of equipment for accomplishing this method principle are described in, for example, Great Britain patent No. 1,026,329, U.S. Pat. No. 3,376,124, German patent No. 2,908,136 or German OS No. 3,043,440.
This known method furnishes satisfactory results so long as relatively large granules are produced. In that situation, high flow velocities of the drying gas may be used for fluidizing the bed of material so that the ratio of throughput and equipment size can be kept within technically reasonable limits.
However, in the production of microgranulates, which are increasingly demanded by industry, difficulties in using the known methods arise. In the case of microgranulates, a fluidization of the bed of material sets in even with very low flow velocities of the drying gas; therefore, with equipment of a given size only a fraction of the yield versus large granules can be obtained or considerably larger equipment must be used. Larger equipment, however, cannot produce a uniform flow bed because of bubble formation in narrowly limited regions which always occurs and causes the remaining flow bed to break down.
The method disclosed in German OS No. 2,231,445 seeks to overcome these difficulties by capturing the granulets discharged with a tubular filter. The drying gas and water vapor pass through the filter while the granulets are returned to the fluidized bed of material. In that method, however, the permissible range of flow velocity of the drying gas remains very narrow. With high flow velocities the smallest granules no longer fall back into the bed of material but rather adhere to the filter bags thus making continuous operation difficult.